Booster brake mechanism



Nov. 19, 1946. w,-F, PENRosE i 2,411,458

I BOOSTER BRAKE HECHANISI Filed July 31, 1944 v 2 Sheets-sneetvi vPatented Nov. 19, 1946 BOOSTER BRAKE MECHANISM V William F. Penrose,Newark, N. J assignor to Empire Electric Brake Company, Newark, N. J., acorporationof New Jersey Application .nay 31, 1944, serial No. 541,363

(o1. iso-54.5)

12 Claims. l

'I'his invention relates to power brake mechanisms, and moreparticularly to a booster hydraulic brake mechanism of the type shown inthe copending application of Rudolph J. Klimkiewicz, Serial No. 504,451,iiled September 30, 1943, now Patent No. 2,377,699.

Booster brake mechanisms of the type referred to have been developed forassisting in the application of braking pressures to the hydraulicallyoperated wheels of a motor vehicle. Devices of this character generallyutilize fluid displaced from the master cylinder for taking up playbetween the brake shoes and brake drums, after which pressure is builtup in the system to actuate a motor to boost the pressure applied to thebrakes. Generally, the pressure developed in the master cylinder isadded to the motor-developed pressure to produce a resultant brakingpressure substantially higher than that developed in the mastercylinder. Such mechanisms employ means for providing for a direct ow oi'brake uid from the master cylinder to take up play between thebrakeshoes and the brake drums, and such means is closed to disconnect themaster cylinder from the brake cylinders-upon actuation ci the motor orupon actuation of the manually operated piston which controls actuationof the motor. A

In `such a mechanism the passage or other means which provides for theflow of fluid from the master cylinder direct to the brake cylindersordinarily provides for a'relatively unrestricted flow of brake fluid.However, a rapid operation of the master cylinder can, and frequentlydoes,

' operate the manually controlled piston to actuate the booster motorand to close direct communication between the master cylinder and thebrake cylinders before play has been taken up between the brake shoesand brake drums. To take care of such a situation it is necessary thatthe high pressure cylinder used in such a mechanism be' of such lengthas to permit displacement/from the high pressure cylinder of the iluidnecessary 'to both engage the shoes with the brake drums and to eiectthe maximum braking force in the event direct communication from them-lster cylinder f manually operable piston, thus insuring againstsubstantial movement of such piston until a relatively great volume ofduid has flowed from the master cylinder to the wheel cylinders to takeup play between the brake shoes and the brake and the brake cylinders iscut oi prior to engagement of the brake shoes with the drums.

A n important object of the present invention is to provide novel meansfor preventing substantial operation of the manually operablemotor-controlled piston until all, or substantially all, oi the iiuidnecessary for engagement of the brake shoes with the brake drums hasowed from the master cylinder to the brake cylinders,

Other objects and advantages of the invention will becomey apparentduring the course of the following description.

In the drawings I have shown on embodiment 'of the invention. VIn thisshowing- Figure 1 is an axial sectional view through the booster brakemechanism, the master cylinder and the brake cylinders together withtheir connections with the booster brake mechanism beingdiagrammatically represented,

Figure 2 is a transverse sectional View taken substantially on line 2-2of Figure 1,

Figure 3 lvis an enlarged fragmentary axial sectional view showing thelow pressure cylinder and the flow control device associated therewith,and

Figure 4 is anenlarged side elevation of the flow control valve.Referring to Figure 1, the numerals I0 and ii respectively indicatealined high and low pressure cylinders of a booster mechanism betweenwhich is arranged a booster motor indicated as a whole by the numeralI2. This motor is preferably of the differential fluid pressure operatedtype and comprises casing sections i3 and Il of generally conical shapesecured at their ends to the respective cylinders i0 and il. A pressuremovable unit indicated as a whole by the numeral IB is arranged in themotor and comprises a iiexible diaphragm Il and a diaphragm I8, theperipheral portion of the diaphragm being arranged between the adjacentilanges i9 of the motor casing sections and clamped in position by asplit band 20, the ends of which may be secured together in any suitablemanner, for example, by a wire 2i (Figure 2). The diaphragm l1 issecured to the plate |81 by a vsnap ring or encircling wire 22 (Figure1).

The diaphragm plate I8 is provided with an axial cylindrical piston 25in which is slidably arranged a manually operable piston 26 having abore 2l therethrough for a purpose to be described. A snap ring 28 isarranged on the piston 28 and is slidable into a recess 29 formed in theend of the `piston h5. A ring 39 is arranged against the adjacent end ofthe piston 25 and engages a double-lipped packing cup 3l retained aroundthe adjacent end of the piston 26 by a snap ring 32. The pistons 2 5 and26 are movable toward the right in Figure 1 to build up pressure in anddisplace fluid from the high pressure chamber 83 in the cylinder Il.Such chamber is connected by suitable lines 34 to the wheel y'cylinders35 ofthe vehicle.

' The left hand end of the piston 26 is grooved as at 38 and the innerend of such groove terminates in a shoulder 39. A double-lipped packingcup 40 is arranged in the groove 38 against the shoulder 39. The lefthand end of the cylinder lil forms a low or primary pressure chamber 52in which is threaded a union 43, and a line 46 connects this union to amaster cylinder '45 having a conventional piston (not shown) thereinoperated by a brake pedal d6.

Valve means are provided for controlling pressures formed in thechambers 50 and 5i in the motor on opposite sides of the pressureresponsive unit. The diaphragm plate I8 (Figure l) carries a preferablyintegral cylindrical extension 52 having a, chamber 53 thereincommunicating with one end of a pressure hose or other conduit 54, thishose being connected to a source of atmospheric or super-atmosphericpressure depending upon Wheth'er the motor is to be vacuum or pressureoperated. In the present instance, the conduit 5 leads to a connection55 communicating with the atmosphere externally of the motor casingsection I4, such space preferably beingcovered by a shell 55 housing asuitable air cleaner 5l.

The tubular extension 52 is provided internally thereof with a valveseat 6G shown in the present instance as being engaged by a ball valveBI urged to closed position by a spring 62, one end of such springengaging the ball 5i and the other end engaging a spring seat 63 whichcloses the adjace'nt end of the extension 52 to communication with themotor chamber 5i. The ball Si is adapted to be unseated by a stem Sioperated in a manner to be described.

A second tubular extension 69 is preferably formed integral with thediaphragm plate i8 and has its interior communicating with the motorchamber 5i through a port 59. A valve, shown as a ball lli, isengageable with a seat il to control communication between the motorchambers 59 and 5i, this valve being urged from its seat by a spring 12of lower tension than the spring t2 for a purpose to be described.

The diaphragm plate i8 is provided with lugs i5 engageable with theadjacent end of :the cylinder l to limit movement of the pressuremovable unit to the left as viewed in Figure v1, and a compressionspring 'i5 urges the pressure responsive unit in such direction. Thelugs l and the piston 2t are straddled by a lever it, this lever beingprovided with an opening 'il through which extend the -lugs l5 and thepiston 26. One end i3 of the lever is directly engageable with the ball19. Centrally of its length the opposite sides of the lever arepreferably stamped to provide bulged points 19 engageable with a yokeBil snapped around the piston 26 and fixed against movement to the leftrelative thereto by a snap ring 8l (Figurefl).

The lever 76" controls the movement of the d valves 5i and it, theformer valve controlling the admission of air as stated while the valve10 controls communication between the motor chambers E@ and 5l. Thelatter chamber is in constant communication with a source of lower pres-In such prior constructions as that shown in Y the said Patent No.2,377,699 to Klimkiewicz, fluid displaced from the master cylinderoperates a piston such as the piston 2S to operate a valve lever, suchas the lever 76, to energize the motor and thus utilize the powerthereof for assisting the manually operated pistons for generatingpressure in the high pressure side of the system leading to the brakecylinders. Before such motor actuation takes place, it is highlydesirable that fluid from the master cylinder first ow through the bore2l to operate the brake cylinders 35 to engage the brake shoes with thedrums. Such operation the brake cylinders requires the movement of a fargreater quantity of fluid than is moved or displaced after initialengagement of the brake shoes takes place. After the latter point inthe' operation of the mechanism is reached, pressure will be built up inthe system and the area of the left hand end of the piston 25 in thepresent construction being greater than the area of the right hand endthereof, the piston 26 will then move to the right to effect motoroperation. The power operation then occurring requires the closing ofthe bore 2l. For this purpose, a rod 9U extends through the bore 21 toengage a ball 9i engageable with a. seat 92 formed at the inner end ofan enlarged bore 93 at the right hand end of the piston 26 in Figure 1,movement of the ball out of the bore 93 being prevented by a pin 94. Ina manner to be described, the rod when the parts of the apparatus are inthe olf position shown in Figure 1 occupies the position holding thevalve 9| off its seat, and the space around the rod 90 and around theball 9| whenthe latter is unseated is such as to provide for thesubstantially unrestricted flow oi' fluid from the master cylinderthrough the bore 2l into the chamber 33 to displace therefrom suiiicientiiuid to engage the brake shoes with the lbrake drums.

In prior constructions, the sudden rapid depression of the brake pedalcan displace iiuid from the master cylinder at a rate greater than suchfluid can flow directly through the appara'- tus into the brakecylinders, thus building up in the chamber d2 a false pressuresuflicient to operate the piston 26 to energize the motor I2 before thebrake shoes are initially engaged with the brake drums by fluid flowingthrough the passage 2, chamber 33 and brake lines 34. This false`operation is due to the fluid friction of the elements through whichfluid flows to the brake cylinders, and to the inertia of the brakeshoes against sudden movements. The present device prevents suchoperation. Referringto Figure 3, the end of the piston 26 is providedwith a bore 99 larger than and communicating with the bore 2l into whichextends a control device 99 pressed as at lilii into the adjacent end ofthe union 43 'and having a bore Ii extending therethrough.

attacca Cav A rubber or similar packing ring itt is carried by thecontrol device 99 for sliding engagement in the bore et. The inner endof the device il@ is provided with a slot H33 to facilitate flow offluid past the adjacent end of the rod 90. The end of the piston 2t inFigure .5 is spaced from the adjacent end of the chamber l2 and in thisspace the device 99 is provided with a relatively restricted meteringport Hill affording limited communication between "the bore lili and thechamber i2 when the parts of the apparatus are in the oi A position.'Ihe rod 90 is engaged against the ad- `iacent end of the device 99 whenthe parts are in the position referred to to hold the ball ai off itsseat. Contact between the rod 90 and the device 99 is established whenthe brake parts reach the fully released position, and when the brakesare applied, brake fluid flowing through the bore lll! readily moves theend oi the rod til toward the right as viewed in Figure 3 for the freeow of fluid from the bore lill into the bore 2l.

The operation of the mechanism is as fol lows: vAssuizning thatA thebrakes are completely released with the parts in the positions shown inthe drawings, brake application will take place upon operation of thepedal 36. Such operation displaces fluid from the master cylinder (itthrough the line ri, bore lill (Fig. 3) and bore 21 into the highpressure chamber 33 (Fig. 1) to displace fluid therefrom to move thebrake shoes into engagement with the brake drums. The iiow of fluidreferred to is relatively unrestricted, Whereas there is substantialrestriction in the flow of fluid through the metering port llll (Fig.3). The ow of huid thus displaced from the master cylinder will resultin a positive flow of a much greater volume of fluid through the bore 2lthan through the port lli, the latter flow being insuflicient to causerapid movement of the piston 26. The flow of iluid through port imi willresult in very slow movement of the piston 2t through which motorenergization is effected. The relatively unrestricted ilow of fluidthrough bore 27 causes positive engagement of the brake shoes with thebrake drums before substantial operation of the piston 26 occurs. andconsequently before any substantial operation oi the motor l2 takesplace, the latter operation being referred to below.

Whether any motor energization taires place prior to initial engagementof the brake shoes with the brake drums depends upon the rate ofdisplacement of iluid from the master cylinder t5. Assuming that theflow capacity of the bore 2l is sufficient under a given ,operatingcondition to engage the brake shoes with the drums prior to motorenergization, contact of the shoes with the drums will be immediatelyfollowed by an increase in pressure in the entire hydraulic system. Thepressure increase will result in a more rapid now of iiuid through theport itil which will be followed by a more rapid movement of the piston2li.

Any substantial movement, of the piston from its normal or off positionwill result in operating the valve mechanism to energize'the motor it.The engagement oi. the member t@ (Figures l and 2) with the points iiiof the lever I6 will tend to cause bodily movement of this lever towardthe right. The spring G2 being of y greater tension than the relativelyweak spring l2, the lever 'i6 will ulcrum at its point of engagementwith the pin and the lower end of the lever will seat the valve lil.Ver; little move ment of the lever iii is required for this opera tionsince the lower end oi the lever 'will move twice the distance of thepiston Prior to the operation referred t@ 'the motor i2 will be vacuumsuspended, that is, balanced subatmospheric pressures will exist in themotor chambers 50 and 5i. The seating of the valve 'i0 closescommunication between the motor chambers, and since the lower end of thelever will be xed against further movement toward the right, continuedslight movement of the pis ton 26 `vill move the upper end of the leverit to unseat the ball tl, thus permitting air to dow past the valve lilinto the motor chamber 5B. The higher pressure thus established in themotor chamber till will effect movement of the pressure movable unit ittoward the right t0 sirniarly move the piston 25. If the movement oi thepiston 2li stops, a very slight additional moven ment of the pressuremovable unit l@ will release the pressure of the upper end of the leverfrom the pin @t and the motor operation also will stop. Any tendency forthe pressure movable unit to move too far will result in cracking thevacuum valve 'it to exhaust some of the air from the motor` chamber 50to establish the proper differential` motor Ipressures to arrestoperation of ,the motor. It wm be obvious that the pressure movable uniti 6 partakes of a follow-up action with respect to the piston 26.

It will be apparent that movement oi the piston 26 toward the rightthrough a relatively short distance will carry the valve seat S32 intoengage ment with the ball di, the rod tl thus becoming ineffective forholding the ball tl ofi its seat. The chamber it thus will bedisconnected from the chamber t2, and movement of the pistons 25 and 2twill displace huid from the chamber 33 into the brake lines to providethe necessary braking pressures, these pressures being built up partlyby manual operation of the piston tt and partly by power operation ofthe piston 25. The operator thus performs part of the work in buildingup the braking pressures and the foot pedal te is always subjected toreaction pressures exactly proportional to the braking pressures.

Assuming that` the movement of huid through tb line (ifi at thebeginning of the brake operation will have taken place incident to avery rapid brake pedal operation, the iiow of fluid through the portHifi may result in sufcient movement of the piston 26 to causeenergization of the motor. However, the movement of the piston tenecessarily will be relatively slow, and the packing db2 is arranged asufcient distance from the left hand extremity of the piston tti toinsure motor operation at a very slow rate to insure the taking up oi'play between the brake shoes and drums prior to the point at which theleft hand extremity of the piston 2S passes the packing H32. From thispoint on, the bore lili will provide for the relatively unrestrictedilow oi fluid into the chamber d2, By this time, however, the brakeshoes will have -been denitely brought into engagement with the brakedrums.

When the foot pedal iid is released, pressure will drop in the chamberi2 and. the pressure acting against the opposite endvor the piston t6will move this piston toward the left (Fig. l) to release the pressureexerted against the valves iii and lll. 'Ihe closing of the valve tlwill cut ofi the chamber 50 from the atmosphere and the opening of thevalve 'ill will connect the chambers il!) and 5l for the exhaustion oiair from the chamber 50. The spring 16 will return the parts to theirnormal positions, and when the rod 90 engages the device 99, the ball 9|will be unseated to reestablish the communication between the line 44and the brake cylinders. The unseating of the valve 9| when the partsare in the normal or off positions provides for the flow of fluidthrough the bore 2l when brake pedal operation is initiated, as stated.The unseating of the ball 9| also permits the replenishing inthe highpressure side of the system of any leakage of brake fluid which may haveoccurred, and the usual valve (not shown) at the outlet of the mastercylinder 45 will maintain the usual residual pressure throughout thesystem.

It will be apparent that the use of the mechanism shown in Fig. 3positively limits the flow of fluid into the chamber 42 to thus limitmotor-4 energizing operation of the piston 26 until the brake shoes havebeen engaged with the drums. Without such means, sudden rapid brakepedal operation may result in the relatively excessive flow of fluidinto the chamber 42 thus building up a false pressure therein. Suchfalse pressure in the chamber 42 would cause a motor-energizing movementof the piston 26 and such movement of this piston not only will energizethe motor but will resultin the closing of the valve 9| before the brakeshoes have been engaged with the drums. Under such conditions, all ofthe remaining fluid necessary for initial engagement ofthe brake shoeswould have to be supplied solely from the chamber 33 and this could takeplace only by movement of the pistons 25 and 26. Unless the highpressure cylinders of prior devices are made relatively long, therefore,it is possible for the piston elements moving into the high pressurechamber to reach their limits of movement prior to a full brakeapplication. To insure the proper operation of Vsuch a mechanism underall conditions, therefore, it is necessary as a practical matten-to makethe pressure cylinders, corresponding to the cylinder relatively long.'Such construction obviously is not necessary in the present device.Therefore, the present device permits the shortening and compacting ofthe mechanism and further insures the performance of the intendedbooster operation of the apparatus.

It is to be understood that the form of the invention herewith., shownand described is to be taken as a preferred 'example of the same andthat various changes in the shape, size and arrangement of parts may beresorted to without departing from the spirit ofthe invention or thescope of the subjoined claims.

I claim:

l. In a booster mechanism for a hydraulic vehicle brake system havingwheel cylinders to'apply the brakes and a pedal-controlled mastercylinder, a booster unit comprising a low pressure cylinder adapted tocommunicate with the master cylinder, a high pressure cylinder adaptedto communicate with the wheel cylinders, a motor having a movable unitincluding a member proi jecting into said high pressure cylinder todisplace fluid into said wheel cylinders upon energization ofsaid motor,a pressure responsive member in said low pressure cylinder operable byfluid displaced from the master cylinder, means operable by saidpressure responsive member for energizing said motor, said booster unitbeing constructed to provide for the substantially unrestricted flow offluid from the master cylinder to saidhigh pressurel cylinder whensaid-pressure responsive member is in its E position, meansv forpreventing such flow of fluid after said pressure responsive member hasmoved a predetermined distance and the pressure in said high pressurecylinder is higher than the pressure in said low pressure cylinder, andmeans co-operating with said pressure responsive member and dependentthereon when the latter is in its off position for limiting the flow offluid from the master cylinder into said low pressure cylinder relativeto the flow of fluid from the master cylinder to said high pressurecylinder when the master cylinder is initially actuated.

2. In a booster mechanism for a hydraulic 'vehicle brake system havingwheel cylinders to apply the brakes and a pedal-controlled mastercylinder, a booster unit comprising a low pressure cylinder adapted tocommunicatewith the master cylinder, a high pressurehcylinder adaptedprojecting into said high pressure cylinder to vdisplace fluid into saidwheel cylinders upon energization of said motor, a pressure responsivemember in said low pressure cylinder operable by fluid displaced fromthe master cylinder, means operable bysaid pressure responsive memberfor energizing said motor, said booster unit being constructed toprovide for the substantially unrelstricted flow of fluid from themaster cylinder to said high pressure cylinder when said pressureresponsive member is in its off position, means for preventing such flowof fluid after said pressure responsive member has vmoved apredetermined distance and the pressure in said high pressure cylinderis higher than the pressure in said low pressure cylinder, and a devicemounted in said low pressure cylinder and communicating with the mastercylinder, such device having a port communicating at all times with saidlow pressure cylinder and of such cross sectional area as to positivelylimit the flow 'of fluid from the master cylinder into said low pressurecylinder relative to the flow of fluid from the master cylinder to saidhigh pressure cylinder when said pressure responsive member is in its oiposition, .said device having a passage normally closed to said lowpressure cylinder by said pressure responsive member when the latter isin its off position and opened to said low pressure cylinder by movementof said pressure responsive member from its off position.

3. In a booster mechanism for a hydraulic vehicle brake system havingwheel cylinders to apply the brakes and a pedal-controlled mastercylinder, a booster unit comprising a low pressure cylinder and a highpressure cylinder arranged in axial alinement andadapted to communicaterespectively with the master cylinder and with the wheel cylinders, adifferential fluid pressure motor having a pressure responsive unit anda member carried thereby and projecting into said high pressure cylinderto displace fluid therefrom upon energization of said motor, a piston insaidlow pressure cylinder movable by fluid displaced from the mastercylinder, a valve mechanism operable by said piston for energizing saidmotor, said booster unit being constructed and arranged to utilize thepressure generated in the master cylinder for assisting said motor ingenerating pressures in said high pressure cylinder, and being furtherconstructed and arranged to provide for the substantially unrestrictedflow of fluid from the master cylinder, and to said high pressurecylinder when said piston is in its ."off" to communicate with the wheelcylinders, a miotor having a movable unit including a member claim 3wherein said metering means comprises a metering device mounted in saidlow pressure chamber, such metering device having a bore therethroughcommunicating with the master cylinder and through which passes theiluid which ilows from a master cylinder to said high pressure cylinder,said metering device having a relatively restricted passagecommunicatying at all times between said bore and said low pressurecylinder.

5. In a booster mechanism for a hydraulic vehicle brake system havingwheel cylinders for applying the brakes and a pedal-controlled mastercylinder, a booster unit comprising a low pressure cylinder and a highpressure cylinder arranged in axial allnement and communicatingrespectively with the master cylinder and with the wheel cylinders, adifferentiaI iluid pressure motor having a pressure responsive unit, auid displacing piston carried by said unit and projecting into said highpressure chamber to displace fluid therefrom upon energization of saidmotor, a control piston having one end arranged in said low pressurecylinder and its opposite end projecting through said nuid displacingpiston to displace fluid from said high pressure chamber, a follow-upcontrol valve mechanism for said motor operable by said control pistonand by said pressure responsive unit, said control piston having a boretherethrough to provide for the substantially unrestricted iiow of fluidfrom the master cylinder to said high pressure cylinder,

means for closing said bore upon'a predetermined movement of saidcontrolpiston from its 10 from upon energization of said motor, apressure responsive device movable by fluid entering said low pressurechamber, said booster mechanism being constructed and arranged toprovide for relatively unrestricted flow of fluid from the fluiddisplacing device to said high pressure chamber when said pressureresponsive device is in its off position, means operative for stoppingsaid low of fluid after said pressure responsive device has been movedand pressure in said high pressure chamber is higher than in said lowpressure chamber, and means co-operating with said pressure responsivedevice and dependent thereon when the latter is in its oil position forpositively limiting the now of fluid from the fluid displacing de viceinto said low pressure chamber relative to said flow of iluid to saidhigh pressure chamber when said. fluid displacing device is initiallyactuated.

8. A hydraulic booster unit comprising a low pressure chamber adaptedfor connection with a hydraulic iluid displacing device,- a highpressure chamber adapted for connection with a device to behydraulically operated, a motor having a fluid displacing elementmovable into said high pressure chamber to displace fluid therefrom uponenergization of said motor, a

off position and when the pressure in said high pressure cylinder ishigher than in said low pressure cylinder, and a metering device forlimiting the flow of fluid into said low pressure cylinder relative tothe ow of fluid through said bore when the master cylinder is initiallyactuated.

6. Apparatus constructed in accordance with claim 5 wherein saidmetering device comprises a stationary member mounted in said lowpressure cylinder and having a bore therethrough directly communicatingwith the bore in said control piston to supply uid thereto directly fromthe master cylinder, said metering device projecting into the adjacentend of said control piston when the latter is in its off position andbeing provided with a relatively restricted port communicating at alltimes with said low pressure chamber, the cross sectional area of saidport being smaller than the cross sectional areas of the bore of saidcontrol piston and said metering device. 4

7, A hydraulic booster unit comprising alow pressure chamber adapted forconnection with a hydraulic iiuid displacing'device, a high pressurechamber adapted for connection with a device to be hydraulicallyoperated. a motor having a iluid displacing element movable into saidhigh pressure chamber to displace fluid therepressure responsive devicemovable by uid entering said low pressure chamber, -sald boostermechanism being constructed and arranged to provide for relativelyunrestricted ow o1' fluid from the fluid displacing device to said highpressure chamber when said pressure responsive device is in its offposition, means operative for closing communication between the uiddisplacing device and said high pressure chamber after said pressureresponsive device has been moved and pressure in said high pressurechamber is higher than in said low pressure chamber, and a deviceprojecting into proximity to said pressure responsive device when thelatter is in its off position and dependent thereon for positivelylimiting the flow of fluid into said lowl pressure chamber relative tosaid ow of fluid into said high pressure chamber when said fluiddisplacing device is actuated.

9. A hydraulic pressure booster mechanism comprising a low pressurechamber and a high pressure chamber in axial alinement and adapted forcommunication respectively with a hydraulic fluid displacing device anda device to be actuated by hydraulic fluid, a diilerential duid pressurevmotor having a pressure responsive structure including a memberprojecting into said high pressure chamber to displace fluid therefrom,a pressure responsive member movable by fluid entering said low pressurechamber, a control valve mechanism operable by said pressure responsivemember to energize said motor, said pressure responsive member extendingthrough said fluid displacing member into said high pressure chamber todisplace fluid therefrom and being provided throughout its length with abore providing for substantially unrestricted now ot fluid therethroughinto said high pressure chamber, means for closing said bore upon apredetermined movement of said pressure responsive member when pressureis higher in said high pressure chamber than in said low pressurechamber, and means for restricting the flow of fluid from said pressuredisplacing device into said low pressure chamber relative to the flow offluid through said bore when said fluid displacing device is initiallyactuated.

11 10. Apparatus constructed in accordance with claim 9 wherein saidlast named means comprises a device mounted in said low pressure chamberand projecting into the adjacent end of said bore, such device beingprovided with a substantially unrestricted passage communicating betweensaid fluid displacing device and said bore, and with a relativelyrestricted port communicting at all times between said passage and saidlow pressure chamber. i

11; In a .booster mechanism for a hydraulic vehicle brake system havingwheel cylinders for applying the brakes and a pedal-controlled mastercylinder, a booster unit comprising a low pressure cylinder and a highpressure cylinder arranged in axial alinement and adapted to communicaterespectively with. the master cylin er and with the wheel cylinders, adifferential d pressure motor having a pressure responsive unit, atubular piston connected to such unit and pro- .jecting into said highpressure chamber to displace fluid therefrom upon energization of saidmotor, a control piston having one end arranged in said low pressurecylinder and its opposite end projecting through said tubular piston todisplace iluid from said high pressure chamber, a follow-up controlvalve mechanism i'or said motor operable by said control piston and bysaid pressure responsive unit, said control piston havstricted ow offluid to said high pressure cylinder, a check valve for said boreseating away from said high pressure cylinder, a rod in said bore, ametering device arranged in said low pressure cylinder and having anaxial passage communicating with the master cylinder, one end of suchdevice extending into one end of said bore to deliver uid directly fromsaid passage to said bore, said metering device having a relativelyrestricted port in constant communication between said passage and saidlow pressure cylinder, one end of said rod being engageable with saidmetering device to unseat said check valve when said control piston isin its oil position.

12. Apparatus constructed in accordance with claim 11 wherein the end ofsaid bore in which said end of said metering device projects is ot v tonhas moved a predetermined distance from its oiT' position.

WILLIAM F. PENROSE.

